280 THE NERVOUS SYSTEM 



Fourth Nerve (Patheticus). 



Origin. This is a purely motor nerve. Its apparent origin 

 is behind the corpora quadrigemina from the valve of Vieussens. 

 The two nerves decussate above this valve. Its deep origin is 

 just below that of the third nerve beneath the aqueduct of Sylvius. 



Course and Distribution. Emerging from the valve of Vie- 

 ussens the nerve winds around the superior peduncle of the cere- 

 bellum and the crusta immediately above the pons, and passes 

 forward near the outer wall of the cavernous sinus to find exit 

 from the cranial cavity by the sphenoidal fissure. Having en- 

 tered the orbit, it runs forward to be distributed to the orbital 

 surface of the superior oblique. In the cavernous sinus it 

 receives fibers from the ophthalmic division of the fifth and from 

 the sympathetic, and occasionally gives off a branch to the lach- 

 rymal nerve. 



Function. It supplies motor power to the superior oblique 

 muscle alone. Remembering the origin and attachment of this 

 muscle it is not difficult to fortell the consequence of lesions of the 

 nerve. The action of the superior oblique is to rotate the ball 

 upon an oblique horizontal axis so that the pupil will look down- 

 ward and outward. This movement cannot be accomplished 

 when the nerve is cut, and the inferior oblique asserts itself un- 

 duly to bring about an opposite effect. The ball cannot accom- 

 modate itself to movements of the head toward the shoulder, and 

 double vision supervenes unless the object be brought in the 

 involuntary line of vision of the affected eye. 



Fifth Nerve (Trifacial, Trigeminus) . 



The fifth is analogous to the spinal nerves (i) in rising by two 

 roots, (2) in having a ganglion on its posterior root, and (3) in 

 having a mixed function. The anterior root is small and motor ; 

 the posterior large and sensory. 



Origin. Its apparent origin is from the side of the pons above 



